Radio antennas have been traditionally installed on vehicles for receiving AM/FM signals. Originally, these vehicular radio antennas were designed to cover AM and FM frequency bands without being concerned about the physical dimensions required for antennas to operate at such bands.
However, over the years the size of these radio antennas has been progressively reduced. This miniaturization addresses current customer demands, while providing a better aerodynamic response and an improved aesthetics. Further, the miniaturization helps reducing the antenna's vulnerability to vandalism compared to conventional whip antennas.
In recent years, the number of radio-communication services, such as AM/FM, telephony GNSS/SDARS or DAB antennas, on the vehicle has significantly increased and combination of several of these services in modules has become a trend. To include further services without increasing the module's volume, which is typically reduced, antenna miniaturization has become a greater concern, especially in antennas of the shark fin type.
Nevertheless, reducing the dimensions of an antenna has consequences in both its bandwidth and impedance.
Current AM/FM miniaturized antennas can have a height of even less than 65 mm, which in terms of wavelength is twelve times lower than a conventional whip antenna design for the FM band. When the volume of an antenna is drastically reduced in terms of wavelength, the expected bandwidth of the antenna suffers a drastic reduction. To make the antenna useful and solve this reduced volume of the antenna in terms of wavelength (λ), a capacitive loading is conventionally used. This problem is not limited to AM/FM antennas and can be extended to other radio antennas operating in VHF or UHF frequency bands.
As mentioned, another consequence of reducing the dimensions of the antenna is that the impedance of the antenna is considerably reduced. Considering an antenna of 65 mm of height (l) operating at 100 MHz (λ=3 m), the impedance of the antenna (Rr) would be lower than 1Ω.
  Rr  =                    40        ·                                            π              2                        ⁡                          (                              l                λ                            )                                2                    ⁢                          ⁢      Rr        ≈          0.5      ⁢      Ω      
This lower impedance has negative effects on the performance of the antenna.
Therefore, it would be desirable in the automotive industry to provide technical means for modifying the impedance of a miniaturized radio antenna in a simple and cost-effective way, while improving current radio antennas' performance.